Stirred Solution/Flow Cell

Potential Hydro- dynamic Stirred Solution/ Flow Cell... 

Convection occurs when a mechanical means is used to carry reactants toward the electrode and to remove products from the electrode. The most common means of convection is to stir the solution using a stir bar. Other methods include rotating the electrode and incorporating the electrode into a flow cell. 

Figure 8. Flow cell for electrochemilumiiiescence measurements (a) glassy carbon electrode (b) sensing layer (c) reagent solution outlet (d) Plexiglas window (e) liquid core single optical fiber (f) stirring bar (g) reagent solution inlet (h) platinum electrode.

Figure 2 shows the essential parts of such an arrangement. There is a (smaller) cell with provisions for measurement of pH, titration, stirring with an inert stirrer, and gas bubbling through the solution. When the solution in this cell has been titrated to the selected pH value, a peristaltic pump takes the solution to the inlet of the flow cell. Before the solution comes to the metal electrode, it passes two thin holes in the bottom of the... 

It is further useful to measure ionic species in stirred or flowing solutions, because the electrode response is then faster, the determination limit is often better than in quiescent solutions and the measurement precision is also improved These improvements apparently result from the effect of solution movement on film diffusion at the electrode surface, which is assumed to be the response-rate determining step [92, 154], An obvious requirement is that the solution velocity and the cell geometry be constant. 

A 1% solution of spray dried powder in water was prepared and gently stirred with a magnetic stir bar until the powder was completely dispersed. The absence of any clumps when the solution was viewed under a microscope was used as an indicator of complete dispersion. A few mL of solution were placed in the chamber of the Microtrac. In the instrument, the solution flows past a laser beam in an optically clear cell. The angle of diffraction of the laser beam is measured and the size of the emulsion calculated. The calculation is based on the principle that the smaller the emulsion size, the larger the angle of diffraction. The instrument gives results on emulsion size and size distribution as well as calculating the surface area of the emulsion. The entire analysis is computerized. 

Figure 5.42 Flow cell for a selective-ion electrode A, sensor electrode B, reference electrode C, solution ground D, sensing membrane E, Teflon sleeve F, Plexiglas cap G, washer H, sample inlet flow /, sample outlet flow J, magnetic stirring bar K, potentiometer L, solution outlet.

Figure 13 Fractional attainment of equilibrium, U(t), for five size fractions of a polydisperse cation exchanger, simultaneously in a stirred flow cell (bottom) during their conversion from the pure A form to the pure B form. For the e Bu-ent solution, the concentrations, Ca/Ca, , for the ions A take up by the ion exchanger, and for the ions B released, Cb/c, , o, as a function of time are given (top). Because the total conversion of the ion exchanger is considered c,, = Flow rate 1.5 mk/s. Solid lines calculated with Eqs. (50), (51), and (52). Dotted lines calculated for a monodisperse system which assumes the panicle radius to be represented by the median radius of the polydisperse mixture.

The instrument illustrated in Fig. 7.6a, similar to that described above, has been applied to the photometric titration of weakly acid drugs In the presence of an immiscible solvent. The essential differences between this Instrument and the previous one lie in the use of a spoiler aimed at minimizing vortex formation arising from the utilization of a stirring bar a burette dispensing the titrant or the washing solution and a triple layer of filter paper on the Teflon membrane to allow It to be traversed by the aqueous phase which, in turn, is propelled to the flow-cell by means of a peristaltic pump [5]. 

Stirring of the sample at a constant temperature is essential in the case of highly intense irradiation. A thermoelement in the cell measures an increase in temperature from 20 0 to more than 35 C by long irradiation. The cell compartment permits the use of a flow cell, which can be used to circulate an externally irradiated solution, for example by a las. This extended setup (see Section 4.6) has been used in the examination of the photostability of laser dyes by combined transmittance and las -power measurements [52]. 

The cell design should aim to meet the process requirements in the simplest (and cheapest) way. Hence separators and even solution flow or stirring should be avoided if at all possible. 

Conventional stirred filtration cells or a specially designed radial-flow cell equipped with a pump can be used. Membranes made of polysulfone, polyamide, cellulose, etc. are suitable. The essential parameters are the molecular mass exclusion rate in wide pH ranges (1-10) and an appropriate permeate flow rate (1-10 ml min ), retentate volume (2-10 ml), and gas pressure (300 kPa is a suitable pressure in most cases). A nominal exclusion rate of lOkgmoH has been shown to be convenient for polymers having a molecular mass between 30 and 50 kg moH. A polymer concentration of 1 % (w/v) in the cell solution is most appropriate for both retention of elements and their subsequent determination in the retentate. 

We have attempted to give supported results in a form appropriate for comparison with unsupported ones by considering full-cell conditions. The transition problems discussed above only occur for unstirred (liquid) electrolytes or for solid electrolytes. When a stirred solution or rotating electrode with lanfinar flow is employed, the I which appears in Z/> and Zw expressions is replaced by 5n, where 5n is the thickness of the Nemst diffusion layer. It decreases as the frequency of rotation of a rotating electrode increases and the experiment is always carried out for conditions where 5 1. 

The concepts of an equivalent, stagnant boundary layer where transport is only by diffusion and of a mass transfer coefficient are equally applicable and useful to the understanding of all stirred and flowing solution cells. 

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